The present invention refers to a method and a system for starting up a flowline suitable for conveying hydrocarbons for offshore production.
According to previously known methods, the flowline, a rigid-type pipeline for example, is first extended over the seabed and connected to an oil production wellhead or to a multiplicity of wellheads and its joint end is then connected to the free end of a subsea riser extended in a catenary, which can itself be rigid. This subsea riser is intended for linking to a surface installation, such as a platform for example.
To make this connection, a known method involves connecting horizontally the subsea riser to the joint end of the flowline, essentially parallel to the seabed surface.
After making the connections, flowline start-up involves conveying hydrocarbon, which arrives at the wellhead, through the flowline and the subsea riser up to the surface installation.
The subsea riser is therefore placed completely on the seabed before making the connection using remote-controlled robots (Remotely Operated Vehicle or ROV), which position themselves either on the flowline for pulling the subsea riser with a cable or on the free end of the subsea riser for drawing it as far as the joint end of the flowline. The other end of the subsea riser is then raised up to the surface installation.
Such a method is notably illustrated in document U.S. Pat. No. 5,975,803. However, other known methods allow the same result to be achieved.
The subsea riser therefore has a free end, which is connected to said joint end of said flowline, a first section, which extends over the seabed from this free end and a rising section, which extends asymptotically, right below the surface installation. A so-called touch-down zone, which corresponds to a dynamic area of the riser, between the first section and the rising section is able to feature a touch-down point which, in turn, is pulled against the seabed, especially by surface installation movements.
The flow line, for its part, is able to deform longitudinally.
In a rigid pipeline, the hydrocarbon passing through the wellhead is, moreover, hot, which causes expansion and elongation of the flowline. In flexible flowlines, injection of fluid under pressure inside also causes their elongation.
The result of this is that movement of the joint end of the flowline and the first section of the subsea riser is produced over a distance of the order of 1 meter, for example 5 meters.
However, the hydrocarbon pressure is not necessarily constant throughout the flowline and, furthermore, supply to the flowline can cease either because of an incident or to allow performance of a maintenance operation at the wellhead.
There is therefore a drawback in dimensional variations of the flowline associated with heating or cooling of the flowline or a pressure variation, which can then cause not only friction of said first section of subsea riser on the seabed, which can be abrasive, but also variation in the radius of curvature of the subsea riser between this section and the rising section or even displacement of the touch-down point position on the pipeline. These drawbacks contribute to its damage in particular.
According to the prior art illustrated in FIG. 1A, the abovementioned drawbacks are overcome by connecting the riser to the flowline using a flexible pipe. This FIG. 1A shows a riser 1, which extends between a free end anchored in the seabed and a surface installation 2, and a flowline 3, whose joint end 4, also anchored, is connected to the riser 1 by a flexible pipe 5. A first section 6 of the riser 1 is moored to the blocks 7 outside the axis of the flowline 3 such that dimensional variations in the flowline 3 are compensated by deformation of the flexible pipe 5. The latter can, moreover, comprise a rigid U-shaped pipe member capable of deformation.
Implementation of such a system is uneconomical, given the different anchor points to be created.
A problem raised, which the present invention aims to solve, is to propose a method and system for starting up a flowline, which allows the service life of subsea risers to be prolonged by preventing their friction on the seabed and their deformation, at an advantageous cost.